Generally, a hydraulic excavator, typical of construction machines, is largely constituted by an automotive base structure, a revolving structure which is rotatably mounted on the base structure, and a working mechanism as a front part liftably mounted on a front portion of the revolving structure, including a boom, an arm and a front attachment (e.g., a bucket).
An operating arm, such as the boom and arm, of the working mechanism (front part) is formed in a square tubular structure of a square shape in cross-section, for example, by joining together four steel plates, i.e., an upper plate, a lower plate, a right side plate and a left side plate (e.g., as disclosed in Japanese Patent Laid-Open No. H11-21939).
For the purpose of enhancing rigidity and at the same time for reducing weight, operating arms on conventional construction machines of this sort are usually fabricated by the use of an upper plate which is provided with a thin wall portion between right and left thick wall portions. Likewise, a lower plate is provided with a thin wall portion between right and left thick wall portions. A square tubular structure is formed by joining right and left side plates with the thick wall portions of the upper and lower plates by butt-welding. In this way, attempts have been made to guarantee high rigidity to a square tubular structure despite reductions in weight.
Further, according to another prior art, a square tubular structure of an operating arm for a construction machine is formed by a combination of four corner members which are located at four corner portions (corners), and four flat plates joined between the corner members (e.g., as disclosed in Japanese Patent Laid-Open No. 2001-20311).
In this case, in order to prevent concentration of stress in corner portions of the operating arm formed as the square tubular structure, each one of the four corner members located at the corners is formed in a curved (or rounded) L-shape in cross-section beforehand. These corner members are joined with the flat plates afterwards by welding to form the square tubular structure which is square in cross-section.
In this regard, in the case of the first-mentioned prior art, thick wall portions are provided in right and left side portions of upper and lower plates, and right and left side plates are joined with the thick wall portions of the upper and lower plates by butt-welding as mentioned above. Thus, in this case, since the upper and lower plates are not required to have a large thickness in their entire bodies, there is an advantage that the weight of the operating arm can be reduced while guaranteeing a certain degree of rigidity.
However, in the case of the prior art just mentioned, at the time of joining right and left side plates with right and left thick wall portions of the upper and lower plates by butt welding, the right and left side plates have to be placed in a precisely aligned position between the upper and lower plates by the use of an aligning jig of a complicate shape. Further, in this case, there is a problem that the arm has to be assembled by a 3D welding operation which requires enormous labor and time.
Following problems arise especially in case high energy density welding such as laser welding is used for 3D welding. Namely, when joined by 3D welding, gaps are likely to occur between joining surfaces of the upper and lower plates and right and left side plates. If a gap of 0.5 mm or greater exists between joining surfaces, for example, the vicinity of the joining surfaces may come out of a laser irradiation range, failing to form a joint of sufficient strength.
In addition, each one of corner portions in the above-mentioned square tubular structure is constituted by a thick wall portion of the upper or lower plate and a joining portion (a welding portion) of the right or left side plate. Therefore, the welding portion in the respective corner portions are susceptible to residual stress or concentration of stress, and are difficult to ensure sufficient rigidity as an operating arm of a construction machine.
On the other hand, in the case of the second prior art mentioned above, corner portions of the operating arm formed as a square tubular structure are formed by corner members of rounded L-shape in cross-section, which has an advantage of suppressing influences of residual stress and concentration of stress.
However, in the case of this second prior art, the four corner members as well as the four flat plates which interconnect the four corner members are formed of steel plates which are substantially uniform in thickness. Therefore, in this case, it is difficult to satisfy two contradictory demands, i.e., weight reduction and high rigidity of an operating arm. That is to say, there is a problem that the weight of the operating arm as a whole is increased if thick steel plates are used to guarantee high rigidity.
If an operating arm is fabricated by the use of thinner steel plates for the sake of weight reduction, corner members and flat plates have to be butted against each other in a precisely aligned state at the time of joining them together, for example, by 3D welding which requires a great deal of labor and time for alignment of joining parts.